This invention is concerned generally with microimaging and concerned specifically with photolithography using lens arrays.
Microlithography has a long history and has provided benefits to many fields. Those familiar with the very tedius and time consuming "wire-wrap" methods can fully appreciate the power of printed circuit technologies. It has now become possible using microlithography methods, to create several million transistors in an area smaller than a square centimeter. Mechanical devices also have enjoyed spectacular advances. It was recently reported in the Journal of Micromechanical Devices that a fully operational motor that is only tens of microns in width can be made using techniques of microlithography. These advances have made the visions depicted in the fictional movie "Innerspace" where microrobotic machines were injected into humans to perform operations on a microscopic level, into something less of a dream and infinitely closer to a reality. And finally, as will become evident in this disclosure, the advanced methods of microlithography have now been discovered to provide contributions to further advanced methods of microlithography. Projection of microimages is common for the manufacturing processes of microdevices including both electric and mechanical microdevices. These include, but are not limited to: electronic type devices such as micro integrated circuits and flat panel display devices, and physical devices such as surface acoustic wave devices, micromotors and other micromechanical devices.
Modern microlithography processes sometimes require projection of light images (photolithography) having features as small as, or even smaller than one micrometer. Images for microlithography applications having such small features are generally produced with large lenses in photolithography projection machines. It is a necessary result of physics that the smaller the features of an image, the larger is the lens necessary to faithfully reproduce those small features. Many problems such as lens abberrations, thermal stability, limited fields-of-view, extremely large optic elements, uniformity, etc., are encountered when large lenses are designed for photolithography applications. Yet, design demands on lenses for photolithography have been increasingly difficult to realize as state-of-the-art advances in the microdevice sciences have pushed for higher lens performance.
A popular exposure tool for projecting images for photolithographic applications is called a "stepper machine". A stepper machine generally has a very precise large lens with a high numeric aperture. Some stepper machines are capable of faithfully reproducing images with features as small as 0.5 microns. Abberations, limit the useful field-of-view of stepper machines to a circular area of a few centimeters in diameter. Many devices, although being comprised of very tiny elements, necessarily extend over several centimeters in their entirety. For example, a Flat Panel Display (FPD) device may be fifty centimeters on a side being made up of millions of individual pixels that are only fifty microns on a side. To manufacture devices that require images larger than the maximum field size of the stepper machine, several stepper image fields are projected in sequential exposures immediately next to each other. This method requires a displacement of the lens with respect to the device substrate that is being printed, and therefore requires some very sophisticated motion and alignment equipment. The exposure steps and move steps are repeated until the entire surface of device substrate is exposed. In this way, a large area device can be "built-up" with a multiplicity of exposures of a single, area-limited stepper field.
It is very difficult to align two stepper fields together. The alignment accuracy is sometimes required to be a small fraction of the image feature size; as small as 100 nanometers. Even with perfect alignment, adjacent images do not always "communicate" well with each other. This is mainly due to third order aberrations such as "pincushion" distortion. Pincushion distortion gets worse as a function of the cube of the radius of an image point as measured from the lens axis in the image plane. These aberrations occur over the entire field area as geometric image placement errors which further complicate alignment of one field to an adjacent field. Because the field size of a stepper exposure machine is limited, and the alignment of one field to an adjacent field is extremely difficult, it necessarily takes a long time for devices requiring a large area exposures to be built-up from a multiplicity of smaller sub-fields. The time that it takes to perform the process limits the amount of devices that a given machine can produce. This limit is expressed as system "throughput" and a primary disadvantage of stepper machines is their low throughput. It is a further problem in the manufacture of flat panel displays to realize high yield. Sometimes during the manufacture of devices flaws occur that can have a catastophic effect on the performance of the device. A single error caused by field misalignment, which is common in stepping methods, can cause an entire device to be useless. Nevertheless, the "step-and-repeat" method has been the preferred method of microlithography to date for even large devices like FPDs. The use of stepper machines has resulted in unacceptably low throughput and yield problems. Production on a mass scale can only be economical with an exposure technique that can improve both the throughput and the yield that are currently known in the step-and-repeat methods.
Other methods of photolithography have been attempted that address the limited throughput and yield problems. A well known method of photolithography for very large areas is called "contact printing". Contact printing does not suffer from field size limitations, but has very bad contamination problems. It is a problem to replace expensive large area photomasks which need to be replaced frequently due to damage caused by contact. Contact printing therefore is not currently considered a feasible alternative. Still another method of large area exposure devices includes holographic photomasks which do not require lenses to form images. Holographic methods usually combine a photomask and an imaging means into a single device. These methods are quite elaborate and have yet to mature to the stage where production manufacturing is practical. Canon corporation offers lens imaging with scanning in one dimension and stepping in an orthogonal direction. A device produced by MRS Technology, Inc. called a Panel Printer.TM., is a photolithography device which specifically built for very large area photolithography. The device of MRS Technology, Inc. called a "stitching aligner". Although it is an advanced type of stepper, it also suffers from "stitching" type alignment errors. A device developed by Ultratech stepper company employs a mirror for reflective type imaging optics and is quite successful in producing some special advantages such as ultra high resolution. However their Markel-Dyson imager is not suited for large areas. All of the photolithography systems mentioned are deficient in their ability to achieve large field, efficient imagers. They can not accomplish the advantages of the current invention and they are not effective replacements thereof.
A unique optical device with special imaging properties is known as a lens array. Until now, applications were limited to fly's-eye, highly parallel type images having very limited applications. A lens array is an arrangement of a plurality of lenses adjacent to each other in a plane with each of their optical axis perpendicular thereto. Although it is possible to arrange conventional lenses in this manner, certain advantages are realized when the lenses are very small in size sometimes called "microlens arrays". There are several techniques used to create a microlens array, and advances in microfabrication techniques have recently provided very spectacular results. For example, some diffractive type devices are made by: 1) Lincoln Laboratory (MIT) produced by reactive ion etching, having numeric apertures from 0.25 to 0.5 are described in a paper published by Leger, Scott, and Veldkamp, in Applied Physics Letters, vol 52 pages 1771-1773 in the year of 1988; 2) OMRON, Japan: diffractive microlens produced by electron-beam lithography, NA=0.25 published by Aoyama, Horie, Yamashita, "Micro Fresnel lens fabricated by electron beam lithography" in SPIE proceedings 1211, 175-183 (1992). Refractive type microlens arrays are known and are simple to fabricate and the following are examples thereof: 1) NSG America inc. 2-d microlens array produced by photolithography and ion-exchange having a numeric aperture of 0.37. 2) CORNING shperical microlens arrays produced directly from glass by a photolytic process, NA&lt;0.35 published in Applied Optics. vol 27, 476-479 1988. and 3) NPL National Physical Laboratory UK, microlens arrays produced by melting small photoresist islands NA&lt;0.5, lenses range in diameter from 5 microns to 500 microns. Diffractive optical elements are sometimes called "binary optical devices" BODs. Binary optical devices can be designed with optical properties that could not be obtained with conventional refractive type spherical optics. For example, it is easy to approximate a lens of parabolic shape having little or no geometric aberrations found in spherical devices. BODs are known by experts to be used for optical interconnects, aberration correction, computer vision, optical multiplexers, and even microsurgery of the human eye. Uses of BODs in array arrangements are described in a paper published in Scientific America by Veldkamp and McHugh, two of the pioneers of binary optics. However, previous to the present disclosure, it was not anticipated that BODs could be used to make even better microlithography tools. It is now possible to provide superior photolithography exposure tools which use techniques of lens array optical imaging.